CN107660324B

CN107660324B - Primary side start-up method and circuit arrangement for series-parallel resonant power converters

Info

Publication number: CN107660324B
Application number: CN201680030226.3A
Authority: CN
Inventors: T·奎格利
Original assignee: Microchip Technology Inc
Current assignee: Microchip Technology Inc
Priority date: 2015-06-01
Filing date: 2016-06-01
Publication date: 2020-11-06
Anticipated expiration: 2036-06-01
Also published as: WO2016196545A1; JP2018516530A; EP3304712B1; EP3304712A1; US10277130B2; CN107660324A; US20160352231A1; KR20180013941A; TW201707360A

Abstract

A series-parallel resonant power converter includes a primary-side start-up controller and a secondary-side controller, wherein the primary-side start-up controller sends power to the secondary-side controller when power (voltage) is first applied to the series-parallel resonant power converter. The start-up controller starts up the series-parallel resonant power converter using an open-loop start-up technique, wherein the secondary-side closed-loop controller takes over control of the series-parallel resonant power converter when the secondary-side closed-loop controller becomes powered on and started up. During light load or no load conditions, the secondary side controller sends an off-resonance higher frequency or standby code disable (disable) command to the start-up controller. When power needs to be sent to the secondary side of the transformer to charge a secondary side capacitor, the secondary side controller can send an enable encoded command to the start-up controller, where it is detected to allow the start-up controller to operate in a normal manner using the secondary side controller.

Description

Primary side start-up method and circuit arrangement for series-parallel resonant power converters

Related patent application

This application claims priority to commonly owned united states provisional patent application No. 62/169,382, filed on day 1, 6/2015; and to us patent application No. 14/945,729 filed on day 11/19 of 2015; and U.S. provisional patent application No. 62/169,415 filed on day 1, 6/2015; all filed by Thomas Quigley (ThomasQuigley), wherein all of said patent applications are hereby incorporated by reference herein for all purposes.

Technical Field

The present disclosure relates to power converters, and in particular, to start-up controller methods and apparatus for DC-to-DC and AC-to-DC series parallel resonant power converters.

Background

A series-parallel resonant power converter is one in which the load may be in series with a resonant "tank" circuit or in parallel with one of the tank circuit components. A series-parallel power converter comprising two inductors, one of which is the magnetizing inductance of the transformer, and a single resonant capacitor is referred to as an "LLC resonant" power converter. The load is connected in parallel with the magnetizing inductance. An "LCC resonant" power converter adds an extra capacitance in parallel with the magnetizing inductance and the load. An advantage of LLC and LCC power converters is the ability to operate at no-load to short circuit conditions, operate over a wide input voltage range, and achieve Zero Voltage Switching (ZVS) and Zero Current Switching (ZCS) over the overall power converter operating range when operating at normal input voltages above resonance. Power converters, such as DC-to-DC and AC-to-DC, typically have unique circuitry for graceful start-up (soft start) as appropriate and to generate the correct operating voltage bias. Such unique circuitry may require custom integrated circuits and/or proprietary designs, which may increase the cost and shipping schedules of such power converters.

Disclosure of Invention

Therefore, there is a need for a low cost solution to start up an offline series-parallel resonant power converter on the primary side using a conventional low cost Integrated Circuit (IC) solution that does not duplicate the resources of the secondary side controller and minimizes the discrete components required for the primary side electronics.

According to an embodiment, a method for starting up a series-parallel resonant power converter may comprise the steps of: applying a first DC voltage to a primary side start-up controller; switching at least one power switch on and off at a frequency higher than a resonant frequency of a series-parallel resonant circuit using the start-up controller, the series-parallel resonant circuit including a primary winding of a transformer coupleable to the at least one power switch; reducing the on and off frequency of the at least one power switch toward the resonant frequency of the series-parallel resonant circuit, whereby an AC voltage may be generated across a secondary winding of the transformer; rectifying the AC voltage from the secondary winding of the transformer using a second rectifier to provide a second DC voltage for powering a secondary side controller and a load; and transferring control of the at least one power switch from the primary-side start-up controller to the secondary-side controller when the second DC voltage may be at a desired voltage value.

According to a further embodiment of the method, the step of using the start-up controller to switch on and off the at least one power switch may be at a fixed higher frequency. According to a further embodiment of the method, the step of using the start-up controller to turn on and off the at least one power switch may be at a fixed lower frequency. According to a further embodiment of the method, the step of using the start-up controller to switch on and off the at least one power switch may start at a fixed higher frequency and may change to a lower frequency. According to a further embodiment of the method, the step of using the start-up controller to switch on and off the at least one power switch may start at a fixed lower frequency and may change to a higher frequency.

According to a further embodiment of the method, the step of transferring control of the at least one power switch from the primary side start-up controller to the secondary side controller may comprise the steps of: sending a signal from the secondary side controller to the primary side start-up controller when the second DC voltage may be at the desired voltage value; detecting the signal from the secondary side controller using the primary side start-up controller; and controlling the at least one power switch using the detected signal from the secondary side controller.

According to a further embodiment of the method, the second DC voltage may be regulated by the secondary side controller after the primary side start-up controller detects the signal from the secondary side controller. According to a further embodiment of the method, the step of sending a signal from the secondary side controller to the primary side start-up controller may further comprise the step of sending a signal through an isolation circuit. According to a further embodiment of the method, the isolation circuit may comprise an optical coupler. According to a further embodiment of the method, the isolation circuit may comprise a pulse transformer. According to a further embodiment of the method, may comprise the step of applying AC power to a first rectifier for providing the first DC voltage. According to a further embodiment of the method, may comprise the step of measuring the current of the primary winding of the transformer using a current transformer coupled to a current sensing input of the secondary side controller.

According to a further embodiment of the method, may comprise the step of limiting a maximum allowable transformer primary winding current using the primary side start-up controller. According to a further embodiment of the method, the second rectifier may be a synchronous rectifier. According to a further embodiment of the method, the synchronous rectifier may be switched at zero voltage. According to a further embodiment of the method, the synchronous rectifier may be switched at zero current. According to a further embodiment of the method, the at least one power switch may be at least one power Metal Oxide Semiconductor Field Effect Transistor (MOSFET). According to a further embodiment of the method, the series-parallel resonant circuit may comprise one inductor, one capacitor and the primary winding of the transformer in an LLC power converter configuration. According to a further embodiment of the method, the series-parallel resonant circuit may comprise two capacitors, one inductor and the primary winding of the transformer in an LCC power converter configuration.

According to a further embodiment of the method, the following steps may be included: sending a disable signal from the secondary side controller to the primary side start-up controller for disabling operation of the power switch when the series parallel resonant power converter may enter a standby mode; and sending an enable signal from the secondary side controller to the primary side start-up controller for enabling operation of the power switch when the series-parallel resonant power can be returned to an operational mode. According to a further embodiment of the method, the disable signal may comprise a first encoded signal and the enable signal may comprise a second encoded signal, wherein the primary side start-up controller may comprise decoding logic for decoding the first encoded signal and the second encoded signal. According to a further embodiment of the method, the enable and disable signals may be at a frequency higher than the pulsed control frequency from the secondary side controller.

According to a further embodiment of the method, the step of using the start-up controller to turn on and off the at least one power switch may comprise the step of generating a bias voltage from a bias winding of the transformer. According to a further embodiment of the method, the step of transferring control of the at least one power switch from the primary side start-up controller to the secondary side controller may comprise the step of the start-up controller accepting a switching command from the secondary side controller so that the secondary side controller may use the start-up controller to control the at least one power switch to substantially achieve linear voltage regulation. According to a further embodiment of the method, the step of using the start-up controller to turn on and off the at least one power switch may comprise the step of turning on and off the at least one power switch when the start-up controller may be in an open loop mode. According to a further embodiment of the method, the start-up controller may provide over-voltage and under-voltage protection, and maximum current limit through the transformer primary winding. According to a further embodiment of the method, a tertiary winding voltage from the transformer may be coupled to the start-up controller and the start-up controller may be enabled to regulate the secondary side voltage if the secondary side controller fails to operate correctly.

According to another embodiment, a series-parallel resonant power converter may include: a primary side start-up controller coupled to a first DC voltage; at least one power switch coupled to the primary side start-up controller; a transformer having a primary winding and a secondary winding; a series-parallel resonant circuit including a primary winding of a transformer coupleable to the at least one power switch; a current measurement circuit for measuring a current through the primary winding of the transformer and providing the measured primary winding current to the primary side start-up controller; a secondary side rectifier coupled to the transformer secondary winding for providing a second DC voltage; a secondary side controller coupled to the primary side start-up controller and the secondary side rectifier; wherein when the primary side start-up controller can receive the first DC voltage, the primary side start-up controller can begin controlling the turning on and off of the at least one power switch at a frequency above a resonant frequency of the series-parallel resonant circuit, which can include the primary winding of a transformer; whereby current may flow through the transformer primary winding, an AC voltage may be generated across the transformer secondary winding, a second DC voltage from the secondary-side rectifier may energize the secondary-side controller, and the secondary-side controller may take over control of the at least one power switch from the primary-side start-up controller when the second DC voltage may reach a desired voltage level.

According to a further embodiment, the at least one power switch may be at least one power Metal Oxide Semiconductor Field Effect Transistor (MOSFET). According to a further embodiment, the secondary side controller may be coupled to the primary side start-up controller by an isolation circuit and may control the primary side start-up controller. According to a further embodiment, the isolation circuit may be an optical coupler. According to a further embodiment, the isolation circuit may be a pulse transformer.

According to a further embodiment, the start-up controller may comprise: a voltage regulator having an input and an output; an internal bias voltage circuit coupleable to the voltage regulator output; an under-voltage lockout circuit coupled to the voltage regulator output; an overvoltage lockout circuit coupleable to the voltage regulator output; a Voltage Controlled Oscillator (VCO) and logic circuit that can generate a variable frequency control signal; a fixed off-time circuit that can be coupled to the VCO and logic circuitry; a power driver that can be coupled to the VCO and logic circuitry and that can provide the variable frequency control signal to the at least one power switch; an external gate command detection circuit that may be adapted to receive an external control signal, wherein when the external control signal may be detected, the external gate command detection circuit may cause control of the at least one power switch to change from the logic circuit to the external PWM control signal; and a voltage comparator that may have an output coupled to the VCO and logic circuit for detecting an overcurrent through the transformer primary winding.

According to a further embodiment, a blanking circuit may be coupled between the current sensing input and the voltage comparator input. According to a further embodiment, the start-up frequency may be determined by a capacitance value of the capacitor. According to a further embodiment, the slew rate of the start-up frequency may be determined by a resistance value of a resistor. According to a further embodiment, the primary side start-up controller may comprise an open loop Voltage Controlled Oscillator (VCO) and a power switch driver. According to a further embodiment, the secondary side controller may comprise a microcontroller.

Drawings

A more complete understanding of the present invention may be acquired by referring to the following description taken in conjunction with the accompanying drawings, in which:

FIG. 1 illustrates a schematic block diagram of a start-up controller for a series-parallel resonant power converter, according to a specific example embodiment of this disclosure;

FIG. 2 illustrates a schematic block diagram of a series-parallel resonant power converter using the start-up controller shown in FIG. 1, according to a specific example embodiment of this disclosure; and

figure 3 illustrates a schematic frequency-impedance plot of the operation of a series-parallel resonant power converter, according to a specific example embodiment of this disclosure.

While the invention is susceptible to various modifications and alternative forms, specific example embodiments thereof have been shown in the drawings and are herein described in detail. It should be understood, however, that the description herein of specific example embodiments is not intended to limit the invention to the particular forms described herein.

Detailed Description

According to various embodiments of the present invention, a series-parallel resonant power converter may include a primary-side start-up controller and a secondary-side controller, wherein the primary-side start-up controller is used to send power to the secondary-side controller when power (voltage) is first applied to the primary side of the series-parallel resonant power converter. The primary side start-up controller may start-up the series-parallel resonant power converter using an open-loop start-up technique, wherein the secondary side closed-loop controller takes over linear closed-loop control of the series-parallel resonant power converter when the secondary side closed-loop controller becomes powered-on and started-up.

This provides a low cost Integrated Circuit (IC) solution for starting up DC-to-DC and AC-to-DC series parallel resonant power converters using conventional means on the primary side that does not replicate the resources of the secondary side controller and minimizes discrete components on the primary side. A more detailed description of the implementation and operation of a Power Converter, in accordance with the teachings of this disclosure, is provided in commonly owned U.S. patent application No. 14/945,729 entitled "Start-Up Controller for a Power Converter" filed by tomas quaigley on 11/19/2015 and hereby incorporated by reference for all purposes.

Series-parallel resonant power converters comprise a very popular topology in the recent past. These series-parallel resonant power converter topologies provide low cost conversion of medium power (e.g., 150-300 watt power converters) using the inherent Zero Voltage Switching (ZVS) and/or Zero Current Switching (ZCS) of power switching Metal Oxide Semiconductor Field Effect Transistors (MOSFETs) with which they are used. Series-parallel resonant power converters require a control method that is different from the techniques used in Pulse Width Modulation (PWM) controlled power converters (e.g., U.S. patent application No. 14/945,729 referenced above). A controller for a series-parallel resonant power converter generates an almost fixed duty cycle waveform with dead times that are short and remain fixed as the switching period changes. This switching waveform changes in frequency to regulate the output of the series-parallel resonant power converter.

The "LLC" symbol of a series-parallel resonant power converter describes a resonant "tank" circuit configuration, which is a series circuit consisting of a resonant inductor (LR), a resonant Capacitor (CR) and a magnetizing inductance (LMAG) of the output transformer. The load is substantially in parallel with the LMAG. The resonant frequency of the tank circuit is a function of LR and CR when the load is close to a short circuit. The resonant frequency of the tank circuit is a function of (LR + LMAG) and CR when the load is close to open. The controller drives the series-parallel resonant power converter at a frequency greater than a resonant frequency of the tank. At lower frequencies of the range, the tank circuit supplies a lower impedance that allows greater power to be delivered to the load. At higher frequencies of the range, the tank circuit supplies a higher impedance that allows less power to be delivered to the load. By maintaining the switching frequency above the tank's resonant frequency (and maintaining a sufficient "Q" for the tank circuit), the converter naturally achieves zero voltage switching of the power MOSFET switch. The LCC power converter is similar in operation to the LLC power converter, but uses an additional capacitor in parallel with the LMAG and the load.

A start-up method for a series-parallel resonant power converter utilizes a primary side controller (which is activated after application of an AC or DC line voltage) to provide a MOSFET gate drive waveform (which starts at a selected switching frequency above the resonant frequency of the series-parallel resonant power converter, which allows low power to the secondary to establish a secondary side bias). As an option, the start-up method may slowly lower the initially selected switching frequency towards its resonant frequency in an open loop manner. This allows the initial low power to the secondary to increase in a soft start manner until the secondary side controller can be activated and take control of the MOSFET power switch.

Referring now to the drawings, the details of example embodiments are schematically illustrated. Like elements in the drawings will be represented by like numbers, and similar elements will be represented by like numbers with a different lower case letter suffix.

Referring now to figure 1, depicted is a schematic block diagram of a start-up controller for a series-parallel resonant power converter, according to a specific example embodiment of this disclosure. The start-up controller 106 may include a High Voltage (HV) voltage regulator 130, an under-voltage AND over-voltage lockout (UVLO/OVLO) circuit 132, a voltage comparator 134, a fixed blanking time circuit 140, external gate command detection logic 142 (with optional enable/disable detection), a Voltage Controlled Oscillator (VCO) AND control/standby logic circuit 144, a three-input AND gate 146, an OR gate 148, a MOSFET gate driver 150, AND a signal buffer 154.

A capacitor 248 (fig. 2) coupled between the PROG node (pin) of the start-up controller 106 and ground may be used to determine the initial start-up frequency. A resistor value 208 (fig. 2) coupled between the MODE node (pin) of the start-up controller 106 and ground may determine the initial start-up frequency droop rate (no resistor, no frequency droop). The primary current of the transformer 230 (fig. 2) is monitored at the input node C/S (current sense) of the start-up controller 106 (C/S monitors the voltage drop across resistor 214) for use with the voltage comparator 134 and a fixed voltage reference V_REFPeak current protection. The fixed blanking time circuit 140 prevents false over-current trips due to on-current spikes during power switching.

Referring now to figure 2, depicted is a schematic block diagram of a series-parallel resonant power converter using the start-up controller shown in figure 1, according to a specific example embodiment of this disclosure. A series-parallel resonant power converter, generally represented by the numeral 200, may include a primary side bridge rectifier 202 coupled to an AC line power source (not shown), a filter inductor 204 and a filter capacitor 210 with a filter damping resistor 206, a resonant inductor 226 and a resonant capacitor 228,

capacitors

218, 236, 246 and 248;

resistors

208, 214, and 244; transformers 220 and 230 (and current transformer 216), MOSFET power switches 222, 224, 232, and 234; a bridge rectifier 250 coupled to the T1 winding of transformer 230 for primary side biasing; a primary side start-up controller 106, a secondary side controller 238, and a first isolation circuit 240 and a second isolation circuit 242 coupled between the primary side start-up controller 106 and the secondary side controller 238, respectively. The series-parallel resonant power converter 200 provides a regulated voltage to an application load (not shown, but located across V _ OUT and RTN) after startup. The AC line power source may be in a general range from about 85 volts Alternating Current (AC) to 265 volts AC at a frequency from about 47 hertz to about 63 hertz. The embodiments disclosed herein are applicable to other voltages and frequencies, which are contemplated and within the scope of the present invention. A DC source may be used in place of the primary side bridge rectifier 202 coupled to an AC source.

The logic of the VCO and control/standby logic 144 determines when the UVLO/OVLO circuit 132 indicates V_DDIs within the proper operating voltage range, the VCO of the VCO and control/standby logic circuit 144 generates a drive waveform starting at a frequency determined by the capacitor 248 coupled to the PROG node (pin) of the primary side start-up controller 106, and slowly (rate determined by the resistance value of the resistor 208) reduces the drive waveform frequency if the resistor 208 is coupled to its MODE node (pin). When the secondary side controller 238 is activated, it monitors the secondary side voltage at its V/S node (pin). When the output voltage level monitored across the capacitor 236 and at node V/S reaches the desired value, the secondary-side controller 238 may command the start-up controller 106 to stop switching by pulling down the PWMD node (pin) through the isolation circuit 242 (e.g., optical coupler, pulse transformer, etc.). If the secondary side controller 238 decides to reapply power to the secondary of the transformer 230, it releases a logic low at the PWMD node. This release is detected by the VCO and control/standby logic circuit 144 and its VCO (144) drives the gate driver 150, again starting with the highest frequency. This is a hysteresis "burst mode" type of operation, providing only enough power to the secondary to keep the secondary side controller 238 active (low standby power operating state). When the secondary side controller 238 decides to apply power to the load, it may take over control of the gate driver 150 by providing a drive waveform to the gate driver 150 via the pulse node of the primary side controller 106. When an external drive signal is detected at the pulse node using the external gate command detection circuit, gate driver 150 receives a pulse command from buffer 154 (instead of the VCO of slave circuit 144) coupled to the pulse node, as long as U is assertedLVO/OVLO circuit 132 determines V_DDWithin the effective voltage level.

VCO and control/standby logic 144 also monitors the state of current sense comparator 134. The over-current trip level set point is an internal voltage reference V_REFAnd the selection of the current sense resistor 214 value. If an over-current is detected (whether gate driver 150 is commanded internally or externally), logic circuitry 144 will temporarily interrupt the command to gate driver 150. After the interrupt time interval, gate driver 150 will resume being externally commanded, or VCO and control/standby circuitry 144 will then begin providing drive waveforms beginning at a frequency determined by programming capacitor 248.

During offline operation (no load connection), the startup sequence may be as follows:

1) an AC line voltage is applied, producing a DC voltage across capacitor 210.

2) The capacitor 246 is charged via the HV regulator 130. When V at start-up controller 106_DDThe voltage at the node reaches the UVLO threshold of the ULVO/OVLO circuit 132, which is activated.

3) MOSFET gate driver 150 drives MOSFET power switches 222 and 224 through gate drive transformer 220 based on commands from VCO 144. The drive frequency may be based on the value of the frequency programming capacitor 248 coupled to the PROG node and the resistance of the resistor 208 coupled to the MODE pin of the start-up controller 106. The drive waveform begins at a frequency selected by the capacitor 248 and slowly begins to decrease at a frequency based on the resistance value of the resistor 208.

4) MOSFET power switches 222 and 224 drive a resonant tank circuit including inductor 226, capacitor 228, and output transformer 230 (which contains an LMAG), thereby charging capacitor 236. There is no load (other than the secondary side controller 238) across the capacitor 236.

5) When the voltage on the capacitor 236 reaches a sufficient level, the secondary side controller 238 is activated. The secondary side controller 238 may be fully analog or fully digital or a combination of both.

6) Secondary side controller 238 regulates the voltage across capacitor 236 at its V/S node using hysteresisThe voltage in this low power standby mode (no load applied) is sensed. Hysteretic control by turning on and off D from the secondary-side controller 238 coupled to the isolation circuit 242 to the PWMD input of the start-up controller 106_OUTAnd outputting to finish. When D is present_OUTWhen the output is turned on, it causes the PWMD node to go low, which disables the signal from the VCO 144 of the start controller 106 to its gate driver 150. When D is present_OUTWhen turned off, the PWMD node is no longer pulled down to a low logic level, whereby gate driver 150 resumes accepting commands from the VCO of circuit 144, wherein the VCO of circuit 144 resets to the frequency selected by capacitor 248 and begins to go to a lower frequency at a rate determined by the resistance of resistor 208. If the resistor 208 is not utilized (present), the frequency remains fixed and does not go low.

7) When secondary side controller 238 couples the load to the output of power converter 200, secondary side controller 238 takes over gate driver 150 from VCO 144. This is accomplished by providing a gate drive command from the gate one-time output of the secondary-side controller 238 to the pulse input node of the start-up controller 106 via the isolation circuit 240.

8) The secondary side controller 238 may have the following features:

a. the VW/S input node may be used to monitor the secondary winding voltage of the transformer 230. This feature may be used so that the secondary side controller 238 can synchronize the driving of the synchronous drive

synchronous rectifiers

232 and 234 with the correct phase with the waveform polarity from the gate driver 150 of the start-up controller 106.

b. Secondary side controller 238 may drive both

primary side MOSFETs

222 and 224 via isolation circuit 240 to control gate driver 150 and provide two gate drivers for

synchronous rectifiers

232 and 234 on the secondary side of transformer 230.

c. The secondary side controller 238 may monitor the primary current through the transformer 230 (and any cross conduction through the power switches 222 and 224) via the current transformer 216, which is monitored at its C/S node.

The secondary side controller 238 may contain an internal VCO generator (not shown) to generate a variable frequency gate signal that is sent to the start-up controller 106 via the isolation circuit 240. This signal maintains almost 50% duty cycle with a programmable fixed dead time period. This signal varies in frequency (within the effective frequency range) to control the impedance of the resonant tank circuit to regulate the output voltage when under load (linear control of the output voltage). The MOSFET power switches 222, 224, 232, and 234 (commanded by the VCO of the secondary side controller 238) may be turned on and off at Zero Voltage Switching (ZVS) and/or Zero Current Switching (ZCS) inherent to the resonant converter topology.

When the load on the power converter 200 becomes light and approaches a no-load condition, the secondary side controller 238 will no longer be able to regulate the voltage at its V/S node in a linear manner. The secondary side controller 238 would then have to resort to a "burst mode" type of control. By "burst mode" is meant that the PWM signal occurs at a brief moment between the times of no switching activity. If the "time without switching activity" is too long, the start-up controller 106 will assume that the secondary side controller 238 has become inactive and it will switch to the start-up mode. Thus, the secondary-side controller 238 uses the PWMD node of the start-up controller 106 (via the drive isolator 242) to control the duration of the "time without switching activity". When the secondary side controller 238 releases the PWMD port, it can decide whether the PWM signal is generated by the secondary side controller 238 (delivered via the isolator 240) or by the start-up controller 106.

The "sleep" mode is also a type of "burst mode" of operation. The difference is that in the "sleep" mode, the controller enters an internal lower power state, where there is an advantage to not continuously drive the isolator 242 to maintain "sleep" (for much longer periods of time without switching activity, resulting in very low power being drawn from the input source). Another difference is that in the "sleep" mode, the secondary side controller 238 no longer attempts to precisely regulate the voltage on the V/S node of the secondary side controller 238, and instead only loosely regulates the voltage on its V/S node and on the VDD node of the start-up controller 106 so that both

controllers

106 and 238 can maintain their internal lower power states. The secondary side controller 238 either decides to enter the "sleep" mode internally or it is commanded externally by a higher level system controller. Commanding the start controller 106 to "sleep" for the secondary side controller 238, which sends an encoded message through the isolator 242, which latches the start controller 106 to sleep and thereby enables it to remain in "sleep" without continuing to drive the isolator 242, reducing power consumption. There are three ways to exit the "sleep" state (wake up the power converter 200). The first is for the secondary side controller 238 to begin sending PWM commands to the start-up controller 106 via the isolator 240. The second is to have the secondary side controller 238 send a single PWM pulse to the start-up controller 106 via the isolator 240, which commands the start-up controller 106 into a start-up mode. The third is caused by the voltage on the VDD node of the start-up controller 106 decaying below the UVLO threshold, causing the start-up controller 106 to enter the start-up mode. The "sleep" Mode is described in its entirety in commonly owned and co-pending U.S. patent application USSN 15/168,390 entitled "Reducing Power in a Power Converter When in Standby Mode" filed by tomass-quingri on 31/5/2016 and hereby incorporated by reference herein for all purposes.

The aforementioned U.S. patent application USSN 15/168,390 also describes that an encoded "sleep" command from the secondary side controller 238 can be transmitted to the start-up controller 106 via the isolator 240. If the particular design of the power converter 200 allows for embedding a "sleep command" in the PWM signal path via the isolator 240, this allows for the elimination of the isolator 242 and the PWMD node from the start-up controller 106, resulting in reduced cost and complexity of the power converter 200 design without any loss of "burst mode" control features as previously described.

Referring now to figure 3, depicted is a schematic frequency-impedance plot of the operation of a series-parallel resonant power converter, according to a specific example embodiment of this disclosure. Fig. 3 (a) shows the fixed start-up frequency at the high impedance point of the power converter 200 impedance curve. Fig. 3 (b) shows the start-up frequency at the high impedance point of the power converter 200 impedance curve while drifting downward in frequency in an open-loop manner. The graphs being at resonant frequencyThe rate transitions from a function of "CR and LR" (heavy load) to a representation of a collection of frequency impedance plots as a function of "CR and LR + LMAG" (light load). The impedance is lowest at the resonance frequency f 0. Both the LLC converter and the LCC converter are capable of operating in a frequency range above f 0. It may be desirable to operate in a frequency range higher than f0 to achieve Zero Voltage Switching (ZVS). The higher frequency in this range is best for start-up because it provides low power (higher impedance) to the output. The start-up frequency may be determined by the capacitance value of a capacitor 248 coupled to the "PROG" node of the start-up controller 106. MODE allows two methods of startup, one in which the startup frequency remains fixed (as shown in (a) in fig. 3) and the other in which the startup frequency starts high (as determined by capacitor 248 on the PROG) and then slowly decreases the switching frequency in an open-loop manner, which increases the power delivered to the secondary (by decreasing the impedance) during startup as shown in (b) in fig. 3. It should be noted that both the LLC converter and the LCC converter may be below the resonant frequency f₀Or higher than f₀Operates in the frequency range of (1). Above f₀Operating at the range of (1) is advantageous as ZVS. The PROG feature of the start-up controller 106, which sets the start-up frequency, may be at a time when the converter is above f₀Or below f₀Is used in operation in the frequency range of (c). However, if the MODE feature is desired, then a different version of the start-up controller 106 will be required for the converter below f₀Operates in the frequency range of (1). In this case, the MODE feature will slowly raise the switching frequency in an open-loop manner, which increases the power delivered to the secondary (by lowering the impedance) during startup.

Referring back to fig. 2, the inductor 226 and the capacitor 228 constitute a resonant tank circuit, which is the frequency dependent impedance described in fig. 3. This resonant tank circuit provides impedance between the input source of the power converter 200 and the load of the output of the converter. This tank circuit is substantially in series with the transformer 230. Thus, whenever MOSFET 222 or 224 (and its respective synchronous rectifier MOSFETs 232 and 234) are turned on, the voltage across the primary winding of transformer 200 is a reflection of the output voltage of power converter 200, as is the voltage across the tertiary winding of transformer 200 (designated by terminals T1-X and T1-Y). The voltage on the tertiary winding is rectified by bridge rectifier 250 to provide a bias voltage at its VDD port to start-up controller 106. In fig. 1, the voltage shown at the VDD port is monitored by the UVLO/OVLO 132 circuit block of the start-up controller 106. Thus, the start-up controller 106 can monitor the output voltage of the power converter 200 for over-voltage and provide protection. This is an important feature. The start-up controller 106 is an open-loop method of starting up the series-parallel converter and relies on the secondary side controller 238 being activated during start-up and obtaining linear control of the output voltage of the power converter 200 before an output overvoltage occurs. However, if the secondary side controller 238 is not operating properly, the start-up controller 106 may provide over-voltage protection by monitoring the output voltage through transformer 230 cross-coupling and provide hysteretic regulation of the output voltage using the UVLO/OVLO 132 circuit blocks. This method of overvoltage protection is similar to that described more fully in co-pending U.S. patent application No. 14/945,729, previously incorporated by reference herein.

Claims

1. A method for starting up a series-parallel resonant power converter, the method comprising the steps of:

applying a first DC voltage to a primary side start-up controller;

turning on and off, by a gate driver, at least one power switch coupled with a series-parallel resonant circuit, the series-parallel resonant circuit including a primary winding of a transformer, thereby generating an AC voltage across a secondary winding of the transformer;

rectifying the AC voltage from the secondary winding of the transformer using a second rectifier to provide a second DC voltage to power a secondary side controller and a load; and

transferring control of the at least one power switch from the primary-side start-up controller to the secondary-side controller when the second DC voltage reaches a predetermined value; wherein

Performing the turning on and off of the at least one power switch using the start-up controller at a frequency higher than a resonant frequency of the series-parallel resonant circuit;

during startup, decreasing the on and off frequency of the at least one power switch toward the resonant frequency of the series-parallel resonant circuit; and

wherein transferring control comprises providing a drive waveform to a gate driver that controls the at least one power switch.

2. The method of claim 1, wherein the secondary-side controller transmits a control signal to the primary-side start-up controller when the second DC voltage is at a desired voltage value, whereby the start-up controller stops turning on and off at least one power switch.

3. The method of claim 1, wherein a rate at which the frequency is reduced is determined by a resistor coupled with the primary side start-up controller.

4. The method of claim 2, wherein the step of transferring control of the at least one power switch from the primary side start-up controller to the secondary side controller comprises the steps of:

sending a signal from the secondary side controller to the primary side start-up controller when the second DC voltage is at the desired voltage value;

detecting the signal from the secondary side controller using the primary side start-up controller; and

controlling the at least one power switch using the detected signal from the secondary side controller.

5. The method of claim 4, wherein the second DC voltage is regulated by the secondary side controller after the primary side start-up controller detects the signal from the secondary side controller.

6. The method of claim 4, wherein the step of sending a signal from the secondary side controller to the primary side start-up controller further comprises the step of sending a signal through an isolation circuit.

7. The method of claim 6, wherein the isolation circuit comprises an optical coupler.

8. The method of claim 6, wherein the isolation circuit comprises a pulse transformer.

9. The method of claim 1, further comprising the step of applying AC power to a first rectifier for providing the first DC voltage.

10. The method of claim 1, further comprising the step of measuring the current of the primary winding of the transformer using a current transformer coupled to a current sense input of the secondary side controller.

11. The method according to claim 1, further comprising the step of limiting a maximum allowable transformer primary winding current using the primary side start-up controller.

12. The method of claim 1, wherein the second rectifier is a synchronous rectifier.

13. The method of claim 12, wherein the synchronous rectifier switches at zero voltage.

14. The method of claim 12, wherein the synchronous rectifier switches at zero current.

15. The method of claim 1, wherein the at least one power switch is at least one power metal oxide semiconductor field effect transistor.

16. The method of claim 1, wherein the series-parallel resonant circuit comprises one inductor, one capacitor, and the primary winding of the transformer in an LLC power converter configuration.

17. The method of claim 1, wherein the series-parallel resonant circuit comprises two capacitors, one inductor, and the primary winding of the transformer in an LCC power converter configuration.

18. The method of claim 2, wherein the control signal comprises a first encoded signal and an enable signal from the secondary side controller comprises a second encoded signal, wherein the primary side start-up controller comprises decoding logic for decoding the first encoded signal and the second encoded signal.

19. The method according to claim 1, wherein the step of turning on and off the at least one power switch using the start-up controller further comprises the step of generating a bias voltage from a bias winding of a second transformer.

20. The method of claim 1, wherein the start-up controller provides over-voltage and under-voltage protection, and maximum current limit through the transformer primary winding.

21. The method of claim 1, wherein a tertiary winding voltage from the transformer is coupled to the start-up controller and the start-up controller is enabled to regulate a secondary side voltage of the secondary side controller if the secondary side controller fails to operate properly.

22. A series-parallel resonant power converter, comprising:

a primary side start-up controller coupled to a first DC voltage, the primary side start-up controller comprising a gate driver;

at least one power switch coupled to the gate driver of the primary side start-up controller;

a transformer having a primary winding and a secondary winding;

a series-parallel resonant circuit including a primary winding of a transformer coupled to the at least one power switch;

wherein the primary side start-up controller is configured to turn on and off at least one power switch, thereby generating an AC voltage across a secondary winding of the transformer;

a secondary side rectifier coupled to the transformer secondary winding for providing a second DC voltage, wherein the secondary side rectifier is configured to rectify the AC voltage from the secondary winding of the transformer to provide a second DC voltage to power a load and a secondary side controller, and the secondary side controller is coupled to the primary side start-up controller and the secondary side rectifier;

wherein the secondary-side controller is further configured to take over control of the at least one power switch from the primary-side start-up controller when providing the second DC voltage for powering the secondary-side controller;

wherein

When on, during start-up, the primary side start-up controller is configured to turn on and off the at least one power switch at a frequency higher than a resonant frequency of the series-parallel resonant circuit;

during start-up, the primary side start-up controller is further configured to decrease the on and off frequency of the at least one power switch toward the resonant frequency of the series-parallel resonant circuit; and

wherein when control is transferred to the secondary side controller, the secondary side controller is configured to control a gate driver by providing a drive waveform to the gate driver, the gate driver controlling the at least one power switch.

23. The power converter of claim 22, wherein the at least one power switch is at least one power metal oxide semiconductor field effect transistor.

24. The power converter of claim 22, wherein the secondary side controller is coupled to and controls the primary side start-up controller through an isolation circuit.

25. The power converter of claim 24 wherein the isolation circuit is an optical coupler.

26. The power converter of claim 24 wherein the isolation circuit is a pulse transformer.

27. The power converter of claim 22, wherein the start-up controller comprises:

a voltage regulator having an input and an output;

an internal bias voltage circuit coupled to the voltage regulator output;

an under-voltage lockout circuit coupled to the voltage regulator output;

an overvoltage lockout circuit coupled to the voltage regulator output;

a voltage controlled oscillator and logic circuit for generating a variable frequency control signal;

a fixed off-time circuit coupled to the voltage controlled oscillator and logic circuit;

wherein the gate driver is coupled to the voltage controlled oscillator and logic circuitry for providing the variable frequency control signal to the at least one power switch;

an external gate command detection circuit adapted to receive an external control signal, wherein when the external control signal is detected, the external gate command detection circuit causes control of the at least one power switch to change from the logic circuit to the external PWM control signal; and

a voltage comparator having an output coupled to the voltage controlled oscillator and logic circuit for detecting an overcurrent through the transformer primary winding.

28. The power converter of claim 22, further comprising a blanking circuit coupled between a current sense input of the secondary side controller and a voltage comparator input of the start-up controller.

29. The power converter of claim 22 wherein the start-up frequency is determined by a capacitance value of the capacitor.

30. The power converter of claim 29 wherein a slew rate of the start-up frequency is determined by a resistance value of a resistor.

31. The power converter of claim 22 wherein a resistor coupled with a primary side start-up controller defines a rate at which the frequency is reduced.

32. The power converter of claim 22, wherein the secondary side controller comprises a microcontroller.